每个 parent 条件的平均值

Question

我有 parent posts table 和 child votes 与 posts.id 和 votes.post_id 关系。我想计算每个 post 的平均评分，但只计算最近给定的 50 票。我知道如何为所有选票做到这一点：

SELECT T1.`title`, (
    SELECT AVG(`vote`)
    FROM `votes`
    WHERE `votes`.`post_id` = T1.`id`
) AS `average`
FROM `posts` T1
GROUP BY T1.`id`

我知道这可能与子查询有关：

SELECT T1.`title`, (
    SELECT AVG(`vote`)
    FROM (
        SELECT `vote` FROM `votes`
        WHERE `votes`.`post_id` = T1.`id`
        ORDER BY `votes`.`id` DESC
        LIMIT 10
    ) AS T2
) AS `average`
FROM `posts` T1
GROUP BY T1.`id`

但出现错误：Error in query (1054): Unknown column 'T1.id' in 'where clause'。 T1 子查询中无法访问别名。有什么想法吗？

http://sqlfiddle.com/#!9/fb9341/2

Answer 1

好的，您想要选择每个 post 最近 50 行中的行。 Stack Overflow 上有很多针对此类查询的答案，大部分在 greatest-n-per-group or limit-per-group tags. Example: How to SELECT the newest four items per category?

下

一旦你写了那个查询，你就可以像你已经知道如何写的那样放入一个子查询，以获得每个 post.

的 AVG(vote)

回复您的评论：

这就是我的意思：

SELECT T1.title, AVG(V.vote) AS avg_vote 
FROM posts T1 
JOIN (
   SELECT v1.id, v1.post_id, v1.vote
   FROM votes v1
   LEFT OUTER JOIN votes v2 ON v1.post_id = v2.post_id and v1.id < v2.id
   GROUP BY v1.id
   HAVING COUNT(*) < 10
) AS V ON T1.id = V.post_id
GROUP BY T1.id;

给定 SQLFiddle 中数据的输出：

+---------+----------+
| title   | avg_vote |
+---------+----------+
| Title 1 |   5.4000 |
| Title 2 |   4.2000 |
+---------+----------+

为了帮助子查询中的 JOIN，您应该在列 (post_id、id) 上的 votes 上有一个索引。

这是另一个不需要唯一列的解决方案：

SELECT T1.title, AVG(V.vote) AS avg_vote
FROM posts T1
JOIN (
    SELECT *
    FROM (
        SELECT v.*, @r := IF(@p = post_id, @r+1, 1) AS rownum, @p := post_id
        FROM (SELECT @p:=null, @r:=0) AS _init
        CROSS JOIN votes v
        ORDER BY v.post_id, v.id DESC
    ) AS t
    WHERE t.rownum <= 10
) AS V ON T1.id = V.post_id
GROUP BY T1.id;

输出与之前的查询相同。

每个 parent 条件的平均值

Average for each parent with condition

mysql

greatest-n-per-group